22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Monte Carlo simulation of the effect of “hot” atoms on active species production
in high-voltage pulsed discharges
N.L. Aleksandrov1, A.A. Ponomarev2 and A.Yu. Starikovskiy3
1
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia
2
SSC Keldysh Research Center, Moscow, Russia
3
Princeton University, Princeton, NJ, U.S.A.
Abstract: Monte Carlo simulation is used to study energy degradation of high-energy H
and O atoms in CH 4 :O 2 and CH 4 :air mixtures taking into account elastic collisions and
chemical reactions. Based on the simulated results, the effect of high-energy atoms on the
amount and composition of chemically active species produced in high-voltage pulsed
discharges is estimated.
Keywords: Monte Carlo simulation, atom energy degradation, active species production,
high-voltage pulsed discharge
1. Introduction
In the last two decades, the applications of nonequilibrium discharge plasma to ignition and combustion
have been thoroughly studied [1-3]. It was shown that
plasma techniques can lead to ignition delay shortening,
flame stabilization, combustion enhancement and
emission reduction. These effects are due to active species
production and rapid gas heating in discharge plasma and
due to modifying transport processes in flames. In
discharges, chemically active species (atoms, radicals and
excited and charged particles) are created in collisions
between molecules and electrons heated in a strong
electric field.
Atoms and radicals produced in the discharge plasma
possess excessive translational energy (a few
electronvolts) that is lost after several elastic collisions
with neutral particles. In [4], it was shown that, prior to
the energy degradation of “hot” particles, they can be
involved in chemical reactions with high energy
threshold. This leads to an additional production of
chemically active species. The purpose of this work was
to simulate numerically this effect and to calculate the
amount of active species produced in discharge plasmas
taking into account chemical reactions with “hot” atoms
and radicals. The simulation was carried out by a Monte
Carlo method allowing competitive consideration of
elastic and inelastic collisional processes leading to the
translational energy relaxation of particles with excessive
initial energy.
2. The method of simulation
We simulated the energy degradation of H and O atoms
with excessive translational energy in stoichiometric
CH 4 :O 2 and CH 4 :air mixtures at different initial gas
temperatures. It was assumed that “hot” atoms were
produced in a high-voltage pulsed discharge due to
electron-impact CH 4 and O 2 dissociation and due to
dissociative quenching of electronically excited N 2
P-I-13-1
molecules in collisions with O 2 and CH 4 . The motion of
atoms and radicals was simulated one after another
neglecting collisions between them. At the beginning, an
atom was originated with a given translational energy in a
given point. The free-flight time between the atommolecule collisions was stochastically determined using
the null collision technique described in detail, for
instance, in [5]. The molecule velocity before collisions
was stochastically generated on the basis of the
Maxwellian velocity distribution with the gas temperature
T. The type of the collision was determined in a stochastic
way using the cross sections for the corresponding
processes. A new atom velocity after the collision was
calculated taking into account the energy released and
assuming that the center-of-mass velocity of the colliding
particles is unaffected by any binary collision and that the
angle distribution of the ion velocities in the center-ofmass frame is isotropic. We simulated the energy
degradation of a given atom and new atoms and radicals
formed during this process until these particles were
thermalized.
Energy degradation was simulated for H and O atoms.
The processes taken into account during the energy
degradation were elastic collisions with CH 4 , O 2 and N 2
and chemical reactions
(1)
H + O 2 → OH + O,
O + CH 4 → CH 3 + OH
(2)
and
H + CH 4 → CH 3 + H 2 .
(3)
The cross-sections for elastic collisions were taken from
[6] assuming the 12-6 Lennard-Jones intermolecular
potential. The cross-sections for reactions (1) – (3) were
determined on the basis of the hard-sphere model with
energy threshold; the parameters for these cross sections
were adjusted to obtain agreement between the calculated
rates and available data in the literature.
1
3. Calculated results
Fig. 1 shows the mean amount of particles produced by
one “hot” atom during its energy degradation as a
function of its initial energy, E 0 . “Hot” H atoms react
with methane when their initial energy exceeds 0.54 eV.
For higher values of E 0 , the amount of H atoms decreases
with E 0 due to reaction (3) in which CH 3 and H 2 are
generated. When E 0 increases up to 1.4 eV, reaction (1)
becomes important and leads to an increase in the amount
of O atoms and OH radicals.
than the masses of O 2 and CH 4 , whereas the mass of O
atoms is comparable with the masses of O 2 and CH 4 .
Calculations show that the effect of “hot” H atoms on
the additional production of active species in the CH 4 :air
mixture is close to that in the CH 4 :O 2 mixture because the
addition of N 2 does not influences strongly the energy
degradation of H atoms due to a large difference in
masses for H and N 2 .
The estimated energy of “hot” H and O atoms produced
in a high-voltage pulsed discharge in air due to direct
electron-impact dissociation and quenching of
electronically excited states of N 2 by CH 4 molecules is in
the range 0.7 – 3.3 eV [4]. Then, it follows from figure 1
that “hot” H and O atoms, during their energy
degradation, are partially lost in chemical reactions to
form CH 3 , OH and H 2 . As a result, the amount of active
species increases and the composition of active species
changes.
4. Conclusions
Using a Monte Carlo technique, energy degradation of
“hot” H and O atoms was simulated in CH 4 :O 2 and
CH 4 :air mixtures. It was shown that the degradation of H
and O atoms leads to the additional generation of CH 3 ,
OH and H 2 . This affects the total amount and composition
of active species produced in high-voltage pulsed
discharges.
5. Acknowledgements
This work was partially supported by the Russian
Ministry of Education and Science under the program
“5Top100”, by the Russian Foundation of Basic Research
under the projects No. 14-03-31449 and No. 14-08-00400
and by the AFOSR MURI program “Fundamental
Mechanisms, Predictive Modeling, and Novel Aerospace
Applications of Plasma-Assisted Combustion”.
Fig. 1. The mean amount of particles produced during
energy degradation of one “hot” H (a) or O (b) atom in a
stoichiometric CH 4 :O 2 mixture at 300 K as a function of
their initial energy (in the stationary laboratory frame).
From figure 1(b), “hot” O atoms are less reactive than H
atoms at the same initial energy, although reaction (2) has
a low (0.44 eV) energy threshold. The difference in the
behavior of high-energy H and O atoms is associated with
the difference in their masses. In reactions (1) and (3), the
excessive energy of H atoms can be totally spent on the
chemical reaction. In contrast, in reaction (2), most of the
translational energy of “hot” O atoms is associated with
the movement of the center-of-mass of the O-CH 4
colliding system that does not change in collisions. In
addition, the rate of energy degradation of “hot” H atoms
is much lower than the rate of energy degradation of “hot”
O atoms because the mass of H atoms is much smaller
2
6. References
[1] S.M. Starikovskaia, Journal of Physics D: Applied
Physics, 39, R265 (2006).
[2] A. Starikovskiy, N. Aleksandrov, Progress in Energy
and Combustion Sciences, 39, 61 (2013).
[3] S.M. Starikovskaia, Journal of Physics D: Applied
Physics, 47, 353001 (2014).
[4] A.Yu. Starikovskiy, 45th AIAA Plasmadynamics and
Lasers Conference, Paper AIAA 2014-2245 (2014).
[5] S. Longo, Plasma Sources: Science and Technology,
15, S181 (2006).
[6] G.V. Dubrovsky, A.V. Bogdanov, Yu.E. Gorbachev,
I.F. Golovnev, Quasiclassical theory of collisions in
gases, Novosibirks: Nauka (1989).
P-I-13-1